The present invention relates to a coating device, and more particularly to an improvement of a curtain-type coating device with which a curtain of a coating solution is allowed to drop onto a moving support under the force of gravity.
Various coating devices of different constructions used to coat supports with chloridic coating solutions have been extensively employed for manufacture of industrial products. The coating solutions may include magnetic coating solutions, photographic photosensitive coating solutions, pressure-sensitive coating solutions, and heat-sensitive coating solutions.
The term "support" as used herein is not particularly limited; however, it is intended to refer to flexible belt-shaped materials such as plastic films made of polyethylene terephthalate, polyethylene-2, 6-naphthalate, cellulose diacetate, cellulose triacetate, cellulose acetate propionate, polyvinyl chloride, polyvinylidene chloride, polycarbonate, polyimide, or polyamide; sheets of paper; sheets of paper on which .alpha.-polyolefins two to ten in the number of carbon atoms, such as polyethylene, polypropylene, ethylene-butane copolymer, are coated or applied in laminated form; metal foils such as aluminum, copper or tin foils; and belt-shaped materials prepared by forming a preliminary layer or layers on the above-described flexible belt-shaped materials. Typical products are photographic films, photographic printing papers, magnetic tapes, and magnetic discs.
There are available a variety of methods for applying the above-described coating solutions. The coating method the invention concerns is the so-called "curtain-type coating method". FIGS. 5 and 6 show examples of the overall arrangement of a coating device for practicing a curtain-type coating method.
In the coating device shown in FIG. 5, a coating head 6 is arranged above a support 4 which is run at a predetermined speed with the coating head covering substantially the entire width of the support 4. The coating head 6 has a solution pool 2 which extends longitudinally of the head 6 (i.e., in the direction of width of the support), and a slit 3 communicated with the solution pool 2. The slit 3 opens upward. A coating solution 1 is supplied into the solution pool 2 by a coating solution supplying system (not shown). The coating solution 1 thus supplied is caused to flow through the slit 3 to the upper surface of the coating head 6, and allowed to drop substantially in the form of a curtain onto the surface of the support 4, thus forming a layer of the coating solution.
In the coating device shown in FIG. 6, the coating head 6 faces downward so that the coating solution 1 emerging from zs the slit 3 is allowed to drop onto the support 4.
The above-described curtain-type coating method suffers from a difficulty that air may rise between the coating solution 1 and the support 4. More specifically, as shown in FIG. 7, when the coating solution 1 collides against the moving s support 4, the position of the collision, namely, the meeting point 5 where the front end of the curtain of coating solution 1 meets the support 4, is shifted in the direction of conveyance of the support 4, and at the same time air 7, being pulled by the support 4, is forced into the layer of coating solution formed on the support 4, thus forming defects 8 therein.
The severity of this difficulty increases as either the coating speed is increased or the thickness of the coated film is decreased, thus causing a serious problem which obstructs not only increasing the range of thicknesses allowable for the coated film, but also improvements in the productivity thereof.